1. Field of the Invention
This invention relates to an automatic lens meter, and in particular to a lens meter capable of accurately measuring the amount of eccentricity of the optic axis of a lens to be examined with respect to the optic axis of a measuring optical system.
2. Related Background Art
There is known an automatic lens meter for finding, from the position of the light spot by a light beam transmitted through a lens to be examined inserted in a measuring optical system, the spherical power, the cylindrical power and the cylindrical axis degree of the lens to be examined and a prism component decomposed in X direction which provides the standard of the cylindrical axis degree and Y direction orthogonal thereto.
For example, in U.S. Pat. No. 4,534,645 the rearward focal plane of a first lens and the forward focal plane of a second lens are made coincident with each other and a pin-hole is provided there, the disposed position (inserted position) of a lens to be examined is formed rearwardly of the second lens, four point light sources are provided on a plane conjugate with said disposed position by said first lens and said second lens, a third lens is provided on the side opposite to said second lens with respect to said disposed position, a light-receiving element for image position detection is disposed at a position conjugate with said pin-hole by said second lens and said third lens, and a converting device is provided for converting a signal corresponding to the position of the light source image output from the light-receiving element to the degree and the principal meridians axis degree (the cylindrical axis degree) of the lens to be examined.
In such a lens meter, the deviation, i.e., the amount of eccentricity, between the optic axis of the measuring optical system and the optic axis of the lens to be examined has been indicated by the positional deviation of an optical image actually passed through the lens to be examined and bent by the prism thereof. That is, in the lens meter as disclosed in said U.S. patent, the amount of eccentricity is regarded as corresponding to the prism, and the movement distance (magnitude and direction) of the central position (this is the origin and the X, Y coordinates system is set in the direction in which two of the four optical images are arranged) of the four optical images spreading when a spherical lens as the lens to be examined is placed without eccentricity is the amount of eccentricity, or the coordinates position (by the above-mentioned X, Y coordinates system) at which a certain optical image should lie when there is no eccentricity is found from the degree and the cylindrical power (for which, for example, the X-axis is the standard of angle measurement) of the lens and the distance from the position at which the measured coordinates position should originally lie is the amount of eccentricity.
However, where the lens to be examined is a spherical lens, the prism and the amount of eccentricity exactly correspond to each other, but there has been the disadvantage that a correct amount of eccentricity is not obtained in an astigmatic lens, particularly, a lens called MIXC in which the spherical power is positive and the cylindrical power is negative.